The present invention relates to an amplifier circuit and more particularly to an amplifier circuit appropriate for an output circuit of an amplifying system.
Conventionally, a class B type amplifier circuit is often used as the output circuit of an amplifying system. Because the class B type amplifier has a good efficiency as compared to a class A type amplifier, relatively little heat is produced in operation and it is, therefore, preferred when relatively large power output is required from an amplifier. The maximum efficiency of a class B type amplifier, however, usually does not exceed 70%. Maximum efficiency is usually realized at maximum output and efficiency decreases as the output decreases. However, during normal operation, an amplifier usually operates at much lower output than maximum most of the time. Therefore, when a large output amplifier is required, the heat evolved is large even if a class B type amplifier is used. Thus, it is necessary to design effective means to dissipate the heat which is generated. This design problem is difficult, and in the case of an integrated circuit output amplifier, the heat generation and dissipation problem restricts the permissible maximum amplifier output.
A proposal for solving the problem is disclosed in U.S. Pat. No. 3,961,280 wherein an amplifier circuit is shown including a switching transistor and an amplifying transistor, both being connected in series in the collector to emitter circuits thereof. A switching transistor switches a power supply from one amplifying transistor to another in accordance with the input signal. Considering the circuit construction, when the number of the switching transistors used is increased, the current capacities of the transistors must be correspondingly large due to the series connection.